Emerging deep ultraviolet light emitting diodes (DUV LEDs) cover the ultraviolet (UV) range down to 210 nanometers (nm), and provide output powers already sufficient for many applications. Additionally, these devices have high modulation frequencies, low noise, flexible form factor and spectral and space power distribution, high internal quantum efficiency, and a potential to achieve high wall plug efficiency. For example, photoluminescence (PL) studies and ray tracing calculations show that the achieved internal quantum efficiency for a 280 nm DUV LED may be quite high, e.g., between fifteen and seventy percent.
However, external quantum efficiency and wall plug efficiency of typical DUV LEDs is below three percent, with the highest efficiencies for 280 nm LEDs and lower efficiencies for LEDs emitting ultraviolet light having shorter wavelengths. Some reasons for the lower external and wall plug efficiencies include very low light extraction efficiency due to internal reflection from the sapphire substrate and sapphire/air interface, and strong absorption in the top low aluminum (AD-content p-type aluminum gallium nitride (AlGaN) and p-type gallium nitride (GaN) layers. The efficiency of the LEDs is further reduced at higher currents and/or generated powers.
In UV LEDs emitting ultraviolet light having a shorter wavelength, the internal quantum efficiency also drops due to materials problems resulting from growth of AlGaN structures with high Al content. Such growth, among other things, is complicated by the low mobility of Al adatoms, which can result in inhomogeneous Al composition and lateral phase separation, as well as high density of threading dislocations and point defects.